Opening scene (depending on the DVD's theatrical or extended versions), shows Needy, a violent inmate, in a correctional facility. She is notorious for being a 'kicker', and is now reflecting during lockdown on past events.
In the small town of Devil's Kettle, Minnesota, the story focuses on two girls who are life-long best friends. One of them is our Anita "Needy" Lesnicky (Amanda Seyfried
), a blond-haired, nerdy, substantially unpopular, "plain Jane" student. The other focus is on Jennifer Check (Megan Fox
), a beautiful black-haired girl who is a cheerleader on the school's flag team. Jennifer's pet name for Needy is Monistat, and Vagisil is, in turn, Needy's nickname for Jennifer. An obnoxious Oriental kid chides Needy for being 'lesbi-gay' but she just points out "sandbox love is forever."
One evening, Jennifer has Needy accompany her to Melody Lane, a local bar, where an indie rock band named Low Shoulder is playing. It seems odd, so unlikely that they would bother to perform at such an insignificant venue, but frontman Nikolai Wolf (Adam Brody) explains that it is a service to their fans. Needy overhears him referring to Jennifer, who is fetching them drinks (getting past the age restriction barrier by playing Hello Titty with the bartender off-camera), as the State Fair Butter Queen, the perennial small-town virgin who loves 'showing it off' but remains a teaser. Obviously, he has unresolved issues with girls. During the performance, the bar catches on fire, but Nikolai gazes on with a smirk, and Needy leads a shocked Jennifer to safety through the toilet window, just barely escaping from being killed in the blaze. Outside, they are accosted by the band who of course made it out okay. However, Needy stands by forlornly as the band members lead a dazed Jennifer to their van, before driving off, with Needy watching as the remains of the bar collapses to the ground in a fiery inferno.
Needy eventually makes it home where she finds a note from her divorced mother who is working swing shift this evening as well.. Needy calls her boyfriend Chip (Johnny Simmons), and explains about the disaster, and has misgivings about Jennifer's fate, fearing the worst. Those guys could be rapists, and that one guy looked especially evil, almost skull-faced, reminding her of a grim-looking petrified tree she saw as a child. Realizing her worst fears, she unexpectedly bumps into Jennifer in her home: clothing torn, bruised and bleeding, looking like a feral zombie-thing just risen from the grave. Things get even stranger, as Needy watches Jennifer tear wolfishly into a raw chicken in her fridge and give an unworldly roar at her, before spewing up a black sickly-looking tarry substance all over the floor. Jennifer further terrorizes Needy when she throws her against a wall asking her if she is scared of her. Jennifer then runs out the front door leaving a bewildered and frightened Needy behind.
The next day, the tragedy of the bar fire has captured the town, and Needy morosely remembers her childhood friend, who then suddenly just reappears looking every inch the hot teen fox again, and impervious to the sorrows of classmates feeling the tragedy, traipses through the somber halls looking like a junior supermodel on a dreamy Valentine date. Flippantly referring to the burn victims as 'a white trash pig roast', her whole manner is "like, get over it!" Jennifer even leads the grieving high school football captain into the nearby woods after school, ostensibly to fulfill his deceased friend's suggestion that they'd have made a 'banging' couple. As a menagerie of forest animals gather to watch, she seduces the big guy, literally leading him like a lamb to the slaughter before she bears down on him with that sexy mouth having turned into a demonic gaping maw. Later on in the day, a highly-concerned teacher at the school finds the captain's grisly remains, and the town is rocked by the additional tragedy. Low Shoulder's song 'Through the Trees' becomes the anthem of hope amidst all the gloom and despair, and obviously, they have chart success, donating 3% to the cause of the victims. Needy questions the paltry effort, and how come they are credited with having rescued survivors while she herself knows that is untrue?
A month passes by, Jennifer begins to look a little pale and gaunt, enigmatically explaining that "it is wearing off", she needs sustenance. As Needy watches, Jennifer catches the eye of the school nerd Colin Gray (Kyle Gallner), and asks him to meet her at her house. She sarcastically says that she's got the DVD of Aquamarine
and that the lead character 'is half-sushi' and that 'she has, like, sex through her blowhole or something', and comments to Needy that the guy is into maggot rock and wears nail polish, so her dick is bigger than his.
The next evening, Needy spends time with her boyfriend Chip, while Colin goes to meet Jennifer... only to find her at a still-in-construction house in a deserted neighborhood where she is in an upstairs room which is lit by various candles. It is here that Jennifer suddenly transforms into a snake-eyed humanoid creature which kills Colin and sups on his flesh, as Needy has a shocking vision that causes her to panic as Chip makes love to her.
Needy leaves her boyfriend's place, and drives away, only to encounter Jennifer walking like an animal across the road, before jumping onto her car's windshield. Needy returns home, only to find Jennifer sleeping in her bed, and, true to the spirit of this black comedy, wearing her Evil Dead T-shirt. Jennifer then kisses Needy, which turns into a hot make-out session, which causes Needy to verbaly snap over what is going on. Needy orders Jennifer to leave, but Jennifer instead confesses what happened to her on the night the local bar burned down....
In a flashback sequence, the band Low Shoulder drove Jennifer out into a forested area, gleefully intending to offer her as a Satanic sacrifice, in exchange for chart success. During the band's discussions, several wondered if Jennifer really was a virgin, and she pretended to be as pure as the morning rain, hoping that they would let her go. Using some instructions they downloaded off the internet, they spoke an incantation to the Devil, before stabbing her with a Bowie knife, which was then thrown into the notorious whirlpool off the nearby Devil's Kettle Falls.
Back in the present, Jennifer further explains to Needy that she found herself still alive (which Needy doubts), and then wandered away from the site after they had left, and that's when Jennifer stumbled back to Needy's place that night. Jennifer also confides to Needy, that when she is fully replenished with human flesh, she is indestructible. She demonstrates this by stabbing herself in the arm with a pocket knife, and Needy watches as the flesh immediately heals. Jennifer also tells Needy that on that same night after she left Needy's house, she came across the Indian exchange student that had survived the fire, walking along the road, and waylaid him, killing and devouring him and then hid the body in the woods. Needy threatens to call the police, but Jennifer tells Needy that she will just deny everything that she just told her and on top of that, no one will ever believe such a story no matter how Needy tells it. With that, Jennifer jumps out of Needy's second floor window and lands 12 feet down on the ground, completely uninjured, and runs off into the night.
Now more worried than ever, Needy goes to the occult section of their school's library, and begins to do research. Needy soon finds where the band went wrong. Because Jennifer was by no means a virgin, their sacrifice backfired, though it indeed brought them prosperity. But it has also turned her into a succubus, a female demon with a constant need to feed on young men to keep in tip-top condition. Needy grows upset because of the upcoming Spring Formal, and how it seems that Jennifer could very well turn the dance into a bloodbath. She informs her boyfriend Chip about this, but he thinks she should get psychiatric help.
Eventually, the Spring Formal comes about, and Needy goes there to meet Chip. Also in attendance is the band Low Shoulder, who have gained a big-time sold-out national tour since that fateful night several months prior. As they take the stage, and Needy does not see Chip anywhere, she bolts from the auditorium. Also, during this time, we see Jennifer ambush Chip in the woods, telling him that Needy was having sex with Colin, and that was why she was so upset with his death. Jennifer wants to hear that Chip prefers her, but his heart belongs to Needy, and she tries to win him over, and she kisses him ardently. They walk towards an abandoned mansion with a graffiti-filled indoor swimming pool, where she flips out into a rage when rejected, throwing Chip into the gunky water.
Needy, in her formal pink ball-gown, goes into a headlong barefoot run, desperate to be there before it is too late. She finds Chip in the water, where the demonic Jennifer, in full-blown animalistic mode, has already bitten into him. He is weakened by extensive blood loss. Needy dives into the water to help Chip crawl out while Jennifer closes in for the kill, and Needy sprays her with a can of mace, which makes Jennifer vomit more black fluid at them. As Needy and Chip crawl out of the dirty swimming pool, Jennifer actually levitates herself out of the pool and confronts them. Jennifer and Needy argue, Jennifer threatens to kill her (she doesn't just kill only boys, she "goes both ways!") and Chip, with his last energy, stabs Jennifer through the stomach with a pool net's handle, impaling her, before she escapes through a nearby window. Needy watches helplessly as her beloved Chip, professing his love, "I think I already died before you got here, but I woke up when I heard your voice", dies in her arms. (The DVD's deleted scenes show how our heroine then went, tattered dress, frog slime-filled hair and all, to confront the band who rushed off the stage upon seeing this vision of vengeance, and she collapsed with sheer exhaustion.) Waking up on her bed looking like a wilted flower, she slowly gathers her wits, and then went for her Dad's toolbox and found a boxcutter.
Later that evening, Needy, hell-bent on revenge, breaks into Jennifer's room, where she and her former best friend scuffle. Jennifer levitates and their fight goes midair. Needy yanks the "Best Friends Forever" locket from her former friend's neck, and, crashing down onto her, stabs Jennifer through the heart with the boxcutter, killing her. However, the noise is heard by Jennifer's mother (Carrie Genzel), who finds Needy crouched over her daughter with the bloody knife. Murder!
Following this, Needy is taken to a mental institution. However, the showdown with Jennifer resulted in her getting scratched and bitten. Supposedly, being bitten by Jennifer also caused some of her demonic powers to be transferred to Needy... as well as her evil personality and attitude. Needy becomes vicious, ill-tempered, and anti-social with the orderlies and other residents. Isolated and abandoned by everyone she knows (nobody from her school, not even her own parents, have come to visit Needy at the hospital), she sets a plot in motion to escape.
After a fight with an institution doctor, Needy is isolated in a cell, but that night she manages to use her new demonic powers to levitate herself to an overhead window, where she kicks it out with her near-superhuman strength, before breaking through a nearby chain-link fence and escaping into the night.
As she wanders down a rural road, she finds a watery runoff (which is where the nearby Devil Falls empties out into), along with the knife that was used on Jennifer. Needy picks up the knife. Flagging down a passing vehicle, she tells the driver, an old guy intent on scoring with her (ass, gas, or grass, yet another deleted scene), that she's following a band, and the old rat exclaims that it must be one hell of a band, to which she then firmly, coldly, grimly adds that their last performance will be tonight.
Wouldn't want to be in their shoes!
As the end credits roll, we are treated to amateur video of the band Low Shoulder checking into a fancy hotel after a concert, and candidly doing things in their hotel room (drinking, smoking dope, snorting cocaine, among other debauchery). Eventually, the scene is shown, in cutaways, of the band being savagely murdered by a blurred figure which enters through the front door. We then see police crime scene photos of the dead band members strewn around the hotel room, with the sacrificial knife in the chest of frontman Nikolai Wolf. Just desserts.
The final image of the movie shows security camera footage of a pale and evil-looking Needy walking down the hotel corridor, away from the band's room after killing all of them as revenge for what they did to Jennifer. Needy walks past a number of excitedly scurrying female band groupies as she walks out of the hotel.